You get a tech tee with your medal at the finish. And yes, the medals this year DID look like willies. Especially from the reverse side. Big LOL!
The run along the coast is beautiful and the beach at Gullane looks gorgeous. I wanted to go paddling in it! Waist pack with water bottle would be a good idea - drink the water on offer at the water stations and refill your bottles too if necessary. Edinburgh is usually cold in May but for whatever reason the marathon day has been scorching hot for something like 4 years out of the last 5.
12,500 runners was for the whole event - the half marathon, 10K and kids runs too. I think there were about 5,000 runners doing the full marathon.
Edinburgh city centre is very expensive to stay in but there are some cheap hostels which have a very good reputation if you don't mind dorm type sleeping. Some of them do have private rooms as well. Also there are the usual Welcome Inn type chain hotels on the outskirts that are £29-ish per room. You don't need to spend a fortune if you don't want to.